1. Field of the Invention
The present invention relates to an alarm device capable of performing transmission and reception of a status signal or the like among a plurality of devices.
2. Description of the Related Art
There is provided an alarm device for detecting heat or smoke that is generated in a room or the like and issuing an alarm. Each of such alarm devices performs an alarm operation independently, and also, in some cases, a plurality of alarm devices provided in respective rooms perform the alarm operation in synchronization with one another.
With regard to a transmission system in which the plurality of alarm devices perform the alarm operation in synchronization with one another, there is proposed “a radio transmission system including a plurality of wireless devices, for transmitting a radio signal among the plurality of wireless devices, in which: each of the wireless devices commonly includes at least one of transmission means for transmitting the radio signal and reception means for receiving the radio signal, and a battery for power supply; the wireless device including the transmission means further includes transmission control means for activating the transmission means when a predetermined event has occurred to alternately repeat operations of transmitting the radio signal in a predetermined transmitting period and pausing the transmitting of the radio signal in a predetermined pause period, and for deactivating the transmission means when the predetermined event has not occurred; the wireless device including the reception means further includes timer means for repeatedly counting a constant intermittent receiving interval, and reception control means for deactivating the reception means while the timer means is counting the intermittent receiving interval, and for activating the reception means every time the counting of the intermittent receiving interval by the timer means is finished; and values a and b satisfy a+2b<T and 2a+b>T under a condition of T>a, where a denotes the transmitting period, b denotes the pause period, and T denotes the intermittent receiving interval” (for example, see JP 2008-176515 A (p. 4, FIG. 1)).
In a conventional transmission system, a transmission side device transmits a status signal or the like in a given constant transmission time length, whereas a reception side device performs a reception operation at intermittent receiving intervals.
In such a transmission system, if all the transmission and reception timings of the respective devices coincide with each other, by causing the transmission side device to perform transmission processing in synchronization with a timing at which the reception side device performs the reception operation, necessary information can be transmitted and received to and from each other. As a result, construction of a system becomes remarkably simple. In addition, if all the transmission and reception timings coincide with each other, necessary information can be transmitted and received without repeatedly performing transmission and reception. As a result, current consumption required for the transmission and reception can also be reduced.
However, in many cases, a transmission processing timing and a reception processing timing of a device are determined using such an electronic part as a clock generator. Such an electronic part has a property that a clock frequency changes depending on a surrounding environment (for example, temperature). If the clock frequency changes, the transmission processing timing and the reception processing timing vary from device to device. In this manner, if there occurs a time lag between the transmission processing timing of the transmission side device and the reception processing timing of the reception side device, the reception side device cannot receive a signal.
For example, by making shorter the intermittent receiving interval of the reception side device, the probability of reception may be increased. However, there arises a problem that the current consumption increases due to the reception processing.
Further, by increasing the number of transmissions by the transmission side device, the probability of reception on the reception side may be increased. However, there arises a problem that the current consumption increases due to the transmission processing.
Further, by separately performing communication for synchronization between the reception side device and the transmission side device, the transmission timing and the reception timing may be matched. However, the current consumption increases due to communication processing for the synchronization.
If the current consumption increases as described above, the battery life of a device powered by a battery becomes shorter, which results in imposing inconvenience on a user, such as need for frequent battery replacement.
Further, wireless devices used in Japan need to meet the provisions of the Radio Law in terms of radio properties to be used. In addition, predetermined standards are set for respective intended uses (for example, standard for radio equipment for a radio station of a low power security system (standard of RCR STD-30 by the Association of Radio Industries and Businesses)). Such standards specify a time length of a transmission period, which is a period in which radio signals are allowed to be transmitted continuously, and a time length of a transmission suspension period, which is a period in which radio signals are not allowed to be transmitted. When transmission processing is performed, the transmission processing needs to be performed in compliance with those standards.